1. Field of the Invention
The invention relates to a projection objective for imaging an effective object field arranged in an object surface of the projection objective onto an effective image field arranged in an image surface of the projection objective and a projection exposure apparatus including such projection objective.
2. Description of the Related Art
Projection objectives are, for example, employed in projection exposure apparatuses used for fabricating semiconductor devices and other types of micro devices and serve to project patterns on photomasks or reticles, hereinafter referred to generically as “masks” or “reticles,” onto an object having a photosensitive coating with ultrahigh resolution on a reduced scale.
A projection exposure apparatus for microlithography includes an illumination system for illuminating the mask, arranged in an exit surface of the illumination system, and a projection objective, following the mask, which is designed for creating an image of the pattern of the mask in the image surface of the projection objective, where the object to be exposed is arranged. The exposure radiation provided by the projection objective is incident on the photosensitive substrate within an effective image field at an image-side numerical aperture NA of the projection objective.
For a given image-side numerical aperture NA, the size, shape and position of the effective image field correspond to an “effective geometrical light conductance value” (or “effective etendue”) which is defined herein as the product of the image-side numerical aperture and the radius of a circle having minimum size including the effective image field.
It is generally desired that the projection objective has a sufficient correction status of all imaging aberrations for all field points within the effective image field in order to obtain the desired performance. The aberrations include chromatic aberrations, image curvature aberration, distortion, spherical aberrations, astigmatism etc. Outside the effective image field the correction status of imaging aberrations and/or the brightness may deteriorate such that no useful image is obtained outside the effective image field.
In the manufacture of highly integrated semiconductor devices it is often desired that at least some layers of a three dimensionally structured semiconductor device are produced at conditions where the image-side numerical aperture and the selected operating wavelength λ of ultraviolet radiation are sufficient to obtain a relatively high resolution R=k1 (λ/NA), where k1 is an empirical constant depending on certain process parameters. For example, resolutions R<100 nm may be required for critical layers.
The image side numerical aperture NA is limited by the refractive index of the surrounding medium in image space. In conventional “dry lithography” a space of finite thickness between an exit surface of the projection objective and the image surface is filled with a gaseous medium with refractive index n′≈1. Projection objectives designed for this process are referred to as “dry objectives” and are limited for physical reasons to values NA<1. Corresponding dry processes are well established. In contrast, immersion lithography allows to extend the processes to values NA>1. In immersion lithography the theoretically possible numerical aperture NA is limited by the refractive index of the immersion medium. The immersion medium can be a liquid (liquid immersion, “wet process”) or a solid (solid immersion). Typically, the refractive index of the immersion medium is larger than 1.3.
Projection objectives optimized for high resolution typically employ operating wavelengths λ<260 nm, e.g. λ=248 nm or λ=193 nm. Since there are only few materials sufficiently transparent in that wavelength region (e.g. synthetic quartz glass (fused silica) and crystalline fluorides, such as calcium fluoride) and the Abbe numbers of those materials are close together, the correction for chromatic aberrations (color-correction) becomes difficult.
The high prices of the available materials and limited availability of crystalline calcium fluoride in sizes large enough for fabricating large lenses represent problems. Measures that will allow reducing the number and sizes of lenses and simultaneously contribute to maintaining, or even improving, imaging fidelity are thus desired.
In optical lithography, high resolution and good correction status have to be obtained for a relatively large, virtually planar effective image field. Providing a flat image requires opposing lens powers and that leads to stronger lenses, more system length, larger system glass mass, i.e. larger overall mass of the transparent material used for lenses and other transparent optical elements.
The resolution and image flatness which can be obtained in an imaging process are only two of a large number of criteria to be observed in a projection exposure process. Often it is desired to improve productivity of manufacturing processes involving the projection objective. Specifically, it may be desired for economical reasons to maximize the number of substrates which can be exposed in a given time interval, e.g. to maximize the throughput of the projection exposure process.
One contribution towards obtaining higher throughput is to use a relatively large effective image field.
The article “Optical Lithography—Thirty years and three orders of magnitude” by J. H. Bruning, SPIE vol. 3049, pages 14 to 27, describes the evolution of optical lithography tools in view of conflicting requirements regarding the resolution to be obtained and the rapid increase of substrate size to be exposed in the field of manufacturing integrated circuits. The author describes that industry moved from contact printing to projection printing in the early 1970's as the complexity of integrated circuits increased and defects became critical. After a time where full wafers where printed at 1:1 magnification, the rapid increase in wafer size was accommodated by annular field scanning using 1:1 imaging mirror systems. Further decreased linewidths requirements and overlay budgets forced the introduction of reduction “step-and-repeat” printing of small blocks of chips in the late 1970's. Further demands for smaller linewidths and larger chips have driven optical lithography to shorter wavelength and to scanning the chip in a “step-and-scan” printing mode. In both processes reduction projection objectives have been used, starting with reduction ratios of 10:1 in the mid-1970's and tending to nowadays reduction ratios of 5:1 or 4:1.
Projection objectives for a step-and-repeat process, often denoted as “stepper lens” or “stepper objective”, are typically designed for a quadratic or a low aspect ratio rectangular effective image field. All-refractive (dioptric) projection objectives are often used for a step-and-repeat process. All-refractive projection objectives allow to use an effective image field centered around the optical axis. A circular area on the image side for which the projection objective must be sufficiently corrected has a diameter corresponding to a maximum image field height Y′.
Scanning systems for a reduction step-and-scan process have been developed for cases where the maximum size of the effective image field is not sufficient to cover the desired size of the area to be exposed on the substrate, e.g. the area of the chip on a wafer. Using a slit-shaped effective image field having a high aspect ratio between the widths (perpendicular to the scanning direction) and the height (in scanning direction) allows to expose larger areas on the substrate by successively exposing adjacent areas on the substrate while the mask and the substrate are moved relatively to each other in a scanning direction.
Due to the slit-shaped effective image field the maximum image field height Y′ for which the projection objective must be corrected is smaller than that needed for a low aspect ratio rectangular or a quadratic effective image field having the same width, if an effective image field centered around the optical axis is used.
The maximum image field height Y′ for which the projection objective must be corrected increases significantly if off-axis fields (effective object and image field arranged entirely outside the optical axis) are used. Off-axis fields are typically employed in catadioptric projection objectives where a pupil obscuration is to be avoided.
If the above requirements towards higher resolution, flat and large image field are to be met, projection objectives typically tend to become bulky requiring a large amount of transparent optical material.